[clang] [Clang][Cygwin] Use correct BuiltinVaListKind. (PR #143166)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> I'm not seeing an obvious place for a target-specific va_list test, can you suggest where this would go?

No idea offhand; I tried breaking the existing `getBuiltinVaListKind` for `WindowsX86_64TargetInfo` and running `check-clang`, which caused failures in the following tests:
```
  Clang :: AST/ast-dump-template-json-win32-mangler-crash.cpp
  Clang :: CodeGen/ms_abi.c
  Clang :: CodeGenCXX/ext-int.cpp
```
So some of them may contain a good sample for tests that cover this bit.

https://github.com/llvm/llvm-project/pull/143166